1. Field of the Invention
This invention relates to a method of forming superconductor films. More particularly, this invention relates to a liquid phase epitaxial method of growing single crystal films of high temperature oxide superconductors
2. Description of the Related Art
Since the discovery of superconductivity in 1911, the phenomenon of a material being able to conduct electricity with almost no resistance when the material is cooled to a temperature approaching absolute zero (0.degree. K.) has remained an interesting scientific curiosity having few applications which would justify the expense of maintaining the necessary liquid helium cooled system.
Recently, however, superconducting ceramic materials have been produced which exhibit this phenomenon at much higher temperatures, e.g., in some cases even higher than the boiling point of liquid nitrogen which boils at about 77.degree. K. The ability to produce superconductivity, for example, in a material cooled by liquid nitrogen completely changes the economics which have previously restricted the applications to which superconductivity could be applied.
These new ceramic materials are sometimes referred to as multi-layer perovskite compounds because of the crystallography of the resulting structure. One such group of compounds is referred to as 1-2-3 compounds because of the atomic ratios of 1 atom of an element such as a rare-earth (Lanthanum series) element, e.g., lanthanum or yttrium; 2 atoms of another element such as an alkaline earth metal, e.g., barium or strontium; and 3 atoms of a metal such as copper. The superconducting ceramic also contains from 6.5+to 7- atoms of oxygen which is usually referred to as O.sub.(6.5+x) where x is greater than 0 and less than 0.5, resulting in a chemical formula such as, for example, XY.sub.2 Cu.sub.3 O.sub.(6.5+x), where X represent the first element, e.g., a rare earth; and Y represents the second element, e.g., an alkaline earth metal.
Other ceramic compounds exhibiting such superconductivity are bismuth-strontium-calcium-copper-oxygen, thallium-calcium-barium-copper-oxygen, bismuth-strontium-calcium-lead copper-oxygen, bismuth-strontium-lead-copper-oxygen, and thallium-calcium-barium-lead-copper-oxygen compounds. Another example of a superconducting ceramic, where copper is omitted, is a barium-potassium-bismuth-oxygen compound.
The prevalent method which has been described in the literature to produce this type of superconducting ceramic is to mechanically mix powders of the oxides or carbonates of the respective rare earth, alkaline earth metal, and copper elements in the 1-2-3 structure of the superconductor, calcine the mixture to remove water or other volatiles, and then fire the powder mixture in an oxygen atmosphere at a temperature sufficiently high to produce the desired superconducting phase.
Such methods have been adequate to form ceramic blocks or masses of superconducting material to verify the formation of superconducting materials for purposes of scientific exploration. However, for the formation of useful materials such as, for example, thin films of superconducting material for use in an integrated circuit structure, it would be desirable to form single crystal films of such superconductors, preferably with the growth of the single crystal oriented to provide superconductivity along one or both axes or the plane of the film.
Hiskes, in an article entitled "LPE GROWTH AND CHARACTERIZATION OF MAGNETIC GARNETS GROWN IN BaO-BASED SOLVENTS", published in the Journal of Crystal Growth, 27, (1974) at pp. 287-298, described the growth of magnetic garnets such as Sm.sub.0.3 Y.sub.2.7 Ga.sub.1.2 Fe.sub.3.8 O.sub.12 from a ternary solvent of BaO-B.sub.2 O.sub.3 -BaF.sub.2 on a Gd.sub.3 Ga.sub.5 O.sub.12 substrate.
It would be desirable to produce epitaxial films of superconductors using a liquid phase growth method to produce such films on a substrate, particularly if crystal growth could be made to occur in an oriented direction by orientation of the substrate on which the epitaxial growth occurs.